fix: Remove unsupported env vars and fix slash commands directory structure

Fixes #42 and #44

- Remove BEADS_ACTOR and BEADS_WORKING_DIR from plugin.json env
  (Claude Code only supports ${CLAUDE_PLUGIN_ROOT} expansion)
- Move commands from .claude-plugin/commands/ to commands/ at root
- Rename command files to remove redundant bd- prefix

This fixes MCP server crashes with literal ${PWD} directory errors
and makes slash commands discoverable in Claude Code.

Amp-Thread-ID: https://ampcode.com/threads/T-69426d6c-25ea-4333-8955-5b624001c8c6
Co-authored-by: Amp <amp@ampcode.com>

commands/show.md

@@ -0,0 +1,17 @@
+---
+description: Show detailed information about an issue
+argument-hint: [issue-id]
+---
+
+Display detailed information about a beads issue.
+
+If an issue ID is provided as $1, use it. Otherwise, ask the user for the issue ID.
+
+Use the beads MCP `show` tool to retrieve issue details and present them clearly, including:
+- Issue ID, title, and description
+- Status, priority, and type
+- Creation and update timestamps
+- Dependencies (what this issue blocks or is blocked by)
+- Related issues
+
+If the issue has dependencies, offer to show the full dependency tree.